1. Field of the Invention
The present invention relates generally to thin film magnetic memory devices and particularly to those having magnetic memory cells with magnetic tunnel junction (MTJ).
2. Description of the Background Art
A magnetic random access memory (MRAM) device has been noted as a memory device capable of non-volatile data storage with low power consumption. The MRAM device is a memory device capable of non-volatile data storage using a plurality of thin film magnetic elements formed in a semiconductor integrated circuit and allows random access to each of the thin film magnetic elements.
In recent years, in particular, it has been announced that a thin film magnetic element utilizing magnetic tunnel junction is used as a memory cell to significantly enhance the MRAM device in performance. An MRAM device having memory cells with magnetic tunnel junction is disclosed in technical documents such as “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, ISSCC Digest of Technical Papers, TA7. Feb. 2, 2000., “Nonvolatile RAM based on Magnetic Tunnel Junction Elements”, ISSCC Digest of Technical Papers, TA7. Feb. 3, 2000., and “A 256 kb 3.0V 1T1MTJ Nonvolatile Magnetoresistive RAM”, ISSCC Digest of Technical Papers, TA7. Feb. 6, 2001.
FIG. 15 is a schematic diagram showing a configuration of a memory cell having a magnetic tunnel junction portion (hereinafter simply referred to as an “MTJ memory cell”).
As shown in FIG. 15, the MTJ memory cell includes a tunneling magneto-resistance element TMR having an electric resistance varying with the level of data stored, and an access element ATR operative in a data read to form a path of a data read current Is passing through element TMR. Access element ATR is formed representatively by a field effect transistor and will thus hereinafter also be referred to as an access transistor ATR. Access transistor ATR is coupled between element TMR and a fixed voltage, e.g., a ground voltage Vss.
The MTJ memory cell is associated with a write word line WWL for indicating a data write, a read word line RWL for effecting a data read, and a bit line BL serving as a data line for transmitting an electrical signal corresponding to a level of data stored in reading and writing data.
FIG. 16 shows a concept for illustrating an operation effected to read data from the MTJ memory cell.
As shown in FIG. 16, tunneling magneto-resistance element TMR has a ferromagnetic layer FL magnetized in a fixed, determined direction (hereinafter also referred to as a “fixed magnetic layer”), and a ferromagnetic layer VL magnetized in a direction corresponding to an externally applied magnetic field (hereinafter also simply referred to as a “free magnetic layer”). Fixed and free magnetic layers FL and VL sandwich a tunneling barrier or film TB formed of insulative film. Depending on the level of data written to be stored, layer VL is magnetized in the same direction as layer FL or opposite in direction to layer FL. Layer FL, barrier TB and layer VL together form a magnetic tunnel junction.
In a data read, read word line RWL is activated and access transistor ATR responsively turns on. This allows bit line BL, element TMR, transistor ATR and the fixed voltage (ground voltage Vss) to form a current path passing data read current Is.
Element TMR provides an electric resistance varying with a relative relationship between a direction in which layer FL is magnetized and that in which layer VL is magnetized. More specifically, if layers FL and VL are magnetized in a single direction (or in parallel), the electric resistance of element TMR is smaller than when the layers are magnetized in opposite directions (or in antiparallel).
As such, if layer VL is magnetized in a direction corresponding to data stored, then a variation in voltage that is introduced by current Is in element TMR depends on the level of the data stored. As such, if for example bit line BL is precharged to have a determined voltage and element TMR then has current Is passing therethrough, then by detecting the voltage on bit line BL the data stored in the MTJ memory cell can be read.
FIG. 17 shows a concept for illustrating an operation effected to write data to the MTJ memory cell.
With reference to FIG. 17, in writing data, read word line RWL is inactivated and access transistor ATR is turned off. In that state, a data write current is passed through write word line WWL and bit line BL to magnetize free magnetic layer VL in a direction corresponding to data to be written. Layer VL is magnetized in a direction determined by a data write current flowing through each of write word line WWL and bit line BL.
FIG. 18 shows a concept for illustrating how a tunneling magneto-resistance element is magnetized in writing data to the MTJ memory cell.
With reference to FIG. 18, a lateral axis H (EA) represents a magnetic field applied in tunneling magneto-resistance element TMR at free magnetic layer VL along an easy axis (EA). A vertical axis H (HA) represents a magnetic field acting at layer VL in a direction of a hard axis (HA). Magnetic fields H (EA) and H (HA) each correspond to one of two magnetic fields, respectively, created by electric currents flowing through bit line BL and write word line WWL, respectively.
In the MTJ memory cell, fixed magnetic layer FL has a fixed direction of magnetization along the easy axis and free magnetic layer VL is magnetized in accordance with levels (of “1” and “0”) of stored data in the direction of the easy axis parallel to (or in the same direction as) layer FL or antiparallel to (or opposite in direction to) layer FL. Hereinafter in the present specification R1 and R0 represent levels of electric resistance of element TMR corresponding to the two types of direction in which layer VL is magnetized, wherein R1 is larger than R0. The MTJ memory cell can store 1-bit data (“1” and “0”) corresponding to the two types of direction in which layer VL is magnetized.
The direction in which layer VL is magnetized can be rewritten only when the sum of applied magnetic fields H (EA) and H (HA) reaches a region outer than an asteroid characteristics line indicated in the figure. In other words, if an applied data writing magnetic field has an intensity corresponding to a region inner than the asteroid characteristics line the direction in which layer VL is magnetized does not switch.
As indicated in the asteroid characteristics line, applying to layer VL a magnetic field having the direction of the harder axis can reduce a threshold value for magnetization that is required to switch a direction of magnetization directed along the easy axis.
As shown in the FIG. 18 example, if a point of operation in writing data is designed, then in a MTJ memory cell to which the data is written a data writing magnetic field in the direction of the easy axis is designed to have an intensity HWR. More specifically, data writing magnetic field HWR is obtained by appropriately designing in value a data writing current flowing through bit line BL or write word line WWL. Generally, magnetic field HWR is represented by the sum of a switching magnetic field HSW required to switch a direction of magnetization and a margin ΔH, i.e., HWR=HSW+ΔH.
Furthermore, rewriting data stored in the MTJ memory cell or a direction in which element TMR is magnetized entails passing a data writing current of no less than a predetermined level on both write word line WWL and bit line BL. Thus element TMR has layer VL magnetized parallel or antiparallel to layer FL to accommodate a direction of a data writing magnetic field directed along easy axis EA. The direction of magnetization once written in element TMR, or data stored in the MTJ memory cell, is held in a non-volatile manner until new data is written.
Tunneling magneto-resistance element TMR thus has an electric resistance varying with a direction of magnetization that is rewritable by an applied data writing magnetic field. As such, correlating two directions of magnetization of layer VL of element TMR with levels (“1” and “0”) of data stored allows non-volatile data storage.
As has been described above, a MTJ memory cell to which data is written (hereinafter also referred to as a “selected memory cell”) needs to have an electric field applied from both write word line WWL and bit line BL corresponding thereto, while from write word line WWL and bit line BL a magnetic field leaks and acts on an MTJ memory cell other than the MTJ memory cell to which the data is written (hereinafter also referred to as a “non-selected memory cell”) and the magnetic filed thus introduces magnetic noise for the non-selected memory cell. If such a noise is large, the non-selected memory cell may disadvantageously have data written erroneously.
In particular, a non-selected memory cell in the same row or column as a selected memory cell has a magnetic field of a predetermined intensity applied thereto in either one of the direction of the easy axis or that of the hard axis. Accordingly it is necessary to prevent a magnetic field acting on each non-selected memory cell of a row or column adjacent to a selected row or column from being affected by a magnetic field leaking from write word line WWL of the selected row and bit line BL of the selected column, so that the former magnetic field does not reach a region outer than the asteroid characteristics line shown in FIG. 18.